Your rarest steam achievement.

Fixer-Upper from Blacklight Retribution, 0.06%

Cheating Death from Schein (0,5%)

My favorite rarest achievement I have is from Alan Wake, Meet the Deadline - 1%

Most of my rarest achievements are all 1% :)

Witch Hunter from Talisman: Digital Editon (0,64%). Exequo with that Genie Character Achievement (I don't know what it's called in english....).

Puzzle Pirates - Ultimative Shipwright with 0.03 % . All my rarest achievements are from Puzzle Pirates except for one. If you are trying to get some rare achievements play free mulitplayer games and just keep playing since a lot of those come from grinding which most people don't have the patience for.

Unturned - Experienced (0,19% players have this)

Technically it's "Purist: Complete a mission on the Space Hulk with 4 Terminators in Chaos Rising." (0.36%) in Dawn of War 2: Chaos Rising, probably because it requires to import a save from Dawn of War 2 vanilla and to make deliberate choices during the campaign. But the rare achievement I'm more proud of is "Tau Efficiency: As the Tau Commander, kill all three doppelgangers within 5 seconds of each other on wave 16 of Bloodied Coliseum yourself." (0.59%) that you acquire in Dawn of War 2: Retribution and was pretty hard to get.

The Witcher 3 - Master Marksman (0.18%)

Guns of Icarus: Research and development; 0.32% according to "Rarest achievement showcase"

GTA 5 - Solid Gold, Baby! (0.34%)

Alien Isolation: One Shot - 0.47% of players have this ^_^
A close second is Finders Keepers from Thief, 0.66% :)

Football Manager 2013 - tutorial master 0,50% :D

Mine is Wicked Collection from Trine 2 with 1.3%

APB reloaded Anarchy - Provocateur 0.05 of steam users.

Kinda off since there is a lot of rerollers/hackers in this game.

When the DLC for Saints Row IV came out (from Season Pass), I did "Walking the Dinosaur" and it still said 0% after some hours. Now it would be probably Money, Money, Money from Tabletop Simulator (0,4 %).

Stormbringer from Metal Gear Rising. 0.65%
It's pretty cool I guess.

Action Henk - Taste the Rainbow (0.14%)

Legend from Batman Arkham Origins

Tactical Intervention
Should have taken a flight - 0.02%

Tank Gunnery Gold from Red Orchestra 2 (0.02%)

Witcher 3 - Master Marksman 0.16%

"Hidden", for Nation Red. 0.31% players have it.

Rising Storm/Red Orchestra 2 - Riding Shotgun Gold (Kill 250 enemies with an on-board machine gun) - 0.01% of players have this.

TBH, I farmed bots for most of the rare achievements in RS/RO2. It would take forever trying to acquire them on full servers.

Broken Age: No Retreat - 0.49%